A number of advanced energy storing device technologies have recently been developed, such as metal hydride (e.g., Ni-MH), lithium-ion, and lithium polymer cell technology, which promise to provide high energy generation for a wide range of commercial and consumer applications. In high-energy applications, a substantial number of individual energy storing devices or cells are typically connected in series and parallel to produce higher voltages and current, respectively. Combining cells in this fashion increases power capacity of the energy storing system. By way of example, it is believed that a battery system suitable for powering an electric vehicle will likely have a voltage rating on the order of several hundred volts, and a current rating on the order of several hundred amperes (amps).
In applications in which rechargeable energy storage cells are connected together in series, it is considered desirable to use cells which are equivalent or very similar in terms of electrochemistry and voltage/current characteristics. It is known that undesirable consequences often result during charging and discharging when an energy storage cell within a series string of cells exhibits characteristics that vary significantly from those of other serially connected energy storage cells. One adverse consequence, for example, involves the voltage of an anomalous energy storage cell within the series string, which will rapidly exceed a nominal maximum voltage limit during charging. Such an overvoltage or overcharge condition may damage the cell and significantly reduce the service life of the cell and other cells within the series connection.
It can be appreciated that the characteristics of mass manufactured energy storage cells will deviate to varying degrees from a given set of requirements. Further, cell characteristics, even if considered acceptable at the time of manufacture, will deviate from manufactured specifications at varying rates over time. In order to accommodate subtle and pronounced differences in cell chemistry and performance between serially connected cells, several techniques have been developed to address the adverse effects of cell non-homogeneity that typically arise when charging a series string of cells.
A conventional approach to protecting a rechargeable energy storing device, such as a battery, from an overcharge condition during charging involves controlling the voltage across the battery terminals. While this approach may be useful when employed for a single battery or for multiple batteries connected in parallel, such a method would prove ineffective for individual batteries connected in series, since the voltage of an individual serially connected battery cannot be controlled using such an approach. Certain approaches that involve the use of an under-voltage switch and an over-voltage switch within a charge control circuit are similarly ineffective for protecting an individual battery within a series string of batteries.
Other conventional overcharge protection schemes which employ over-voltage and under-voltage switches suffer from another significant deficiency, particularly when high current and voltage ratings are implicated. The over-voltage and under-voltage switches in such schemes typically must support the full discharge and charge current of the battery system. In high-energy applications which require the production and transport of several hundred amps of current, such switches would likely be prohibitively expensive and difficult to accommodate in a cost-efficient power system design. Moreover, when one of the series connected cells or batteries achieves a fully charged state, a conventional overcharge protection circuit typically interrupts the flow of charge current to all other cells or batteries within the series connection. As such, each of the serially connected cells or batteries is charged to a different potential, thereby resulting in an unbalanced energy storage system.
There is a need in the battery manufacturing industry for an apparatus and method for effectively and safely charging a number of serially connected energy storing devices. There exists a further need for a methodology for balancing the potentials of series connected energy storing devices. The present invention fulfills these and other needs.